To provide protection from the possibility of electrical shock, many appliances provide a connection between their exposed metallic portions and electrical ground. Should this connection fail for some reason, its measure of protection no longer remains available.
Most modern hospital beds include at least some electrical facilities, especially one or more motors to alter its configuration. Because of the possibly prolonged and intimate contact of a patient's skin with the bed frame, the connection to ground represents a particularly important safeguard. Accordingly, the safety of the bed increases where it includes an indication that its connection to ground remains intact.
A. P. Petzon et al.'s U.S. Pat. No. 3,716,876 incorporates, in the circuitry for a bed, a ground-indicating device. However, this simply amounts to a light bulb connected between ground and the side of the transformer receiving the a.c. power.
This device, consequently, suffers from two serious limitations. First, it lacks any ability to indicate when the a.c. voltage source has reversed polarity, with the "hot" wire appearing at the common terminal and vice versa.
Second, all current passing through the bulb, which generally remains lit, enters the ground system. The bulb from each bed would pass at least about one milliampere. In a hospital having several hundreds of beds, the current in the supposedly safe ground system could well exceed onequarter of an ampere, an unacceptably high level.